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Abstract:

A method for producing a capping wafer for a sensor having at least one
cap includes: production of a contacting via extending through the wafer,
and, temporally subsequent thereto, filling of the contacting via with an
electrically conductive material.

Claims:

1-15. (canceled)

16. A sealing glass capping wafer, comprising: an electrical contact pin
extending through the capping wafer; and a first electrical terminal
provided on the electrical contact pin for the electrical contacting of
an electrical terminal of a sensor wafer; wherein an electrical
through-connection is provided by the electrical contact pin and the
electrical terminal.

17. The capping wafer as recited in claim 16, wherein the electrical
contact pin has a second electrical terminal for the electrical
contacting of an electrical terminal of a substrate.

18. The capping wafer as recited in claim 17, wherein the electrical
through-connection of the capping wafer is produced by electroplating.

19. The capping wafer as recited in claim 17, wherein the first and
second electrical terminals one of (i) lie externally on the capping
wafer or (ii) stand away from the capping wafer.

20. A microelectromechanical sensor system comprising: a sealing glass
cap; and a sensor element.

21. The sensor system as recited in claim 20, wherein a solder bump is
provided on an electrical rear side contact of an electrical contact pin
of the sealing glass cap, and wherein an electrical terminal of a
substrate is electrically contacted using the solder bump.

22. A sensor system, comprising: a substrate; and a sensor having a cap,
a sensor element, and an electroplated electrical through-connection of
the cap to the sensor element, wherein the electrical through-connection
is electrically conductively connected directly to an electrical terminal
of the substrate.

Description:

[0001] The present invention relates to a method for producing a capping
wafer for a sensor, in particular a sensor in a motor vehicle. In
addition, the present invention relates to a method for producing a
sensor stack, in particular an inertial sensor stack, having at least one
sensor. In addition, the present invention relates to a capping wafer, to
a sensor having a cap according to the present invention, and to a
configuration of a substrate having a sensor according to the present
invention.

[0002] A sensor, in particular a microelectromechanical sensor (MEMS
sensor), in particular a capacitive inertial sensor for measuring
accelerations and/or rates of rotation, is as a rule packaged in a
plastic housing together with an IC chip that evaluates and pre-processes
a raw sensor signal. Before installing the sensor in the plastic housing,
a sensitive sensor structure is provided with a cap. This is necessary in
order to protect movable sensor structures from subsequent processes,
including for example gelation, and in order to permit the setting of a
desired internal pressure and thus a desired sensor oscillation quality.

[0003] For this purpose, before the separation of a large number of
(capacitive inertial) sensors, a so-called capping wafer is bonded onto
an electrical sensor wafer using sealing glass bonds. This results in a
so-called sensor stack that comprises the sensor wafer and the capping
wafer fastened thereon, forming a large number of sensors bonded fixedly
to one another. Here, the sealing glass is applied in the shape of a
frame around caverns of the capping wafer using a screen printing method.
The sealing glass (glass solder) ensures a hermetic seal between the
capping wafer and the sensor wafer in the region of the caverns.

[0004] The job of the caverns is to keep small a relative change of an
enclosed volume between the capping wafer and the sensor wafer when the
sealing glass is pinched, and to act as a pressure or vacuum reservoir
for the sensor, which will later be separated. The accesses to the
electrical terminals (bonding pads) of each sensor are situated outside
the frame, and in the capping wafer are already recessed in a suitable
manner and thus made accessible. See here for example DE 197 00 734 A1.

[0005] After the separation of the sensor stack, the individual sensors
can be glued into the plastic housing. The electrical connections from
the sensor to the IC chip and the electrical connections from the
electrical terminals of the housing to the bonding pads of the IC chip
are realized using bonding wires. Subsequently, the bonding layers are
gelated and the plastic housing is sealed. Only then is the resulting
sensor element ready to be soldered onto a circuit board. In sensors
having a hybrid construction (two-chip solution having a sensor chip and
an IC chip), such a procedure is required because the electrical
connections, which are high-ohmic and are susceptible to disturbances,
between the (capacitive) sensor and an evaluation electronics system must
be realized in a defined manner, mechanically fixed, and protected.

[0006] U.S. Pat. No. 7,275,424 B2 discloses a manufacturing method for a
sensor stack, as well as a sensor stack, or a manufacturing method for a
capped sensor, as well as a sensor. Here, in a capping wafer caverns are
provided, and outside a cavern electrically insulated vias are provided.
Subsequently, the capping wafer is further processed, by a sealing glass
bonding with a sensor wafer, to form a sensor stack, the sealing glass
being applied onto the capping wafer using a screen printing method, and
the sealing glass bond between the capping wafer and the sensor wafer
being formed subsequently. After this, the vias of the sensor stack are
filled with a doped polysilicon material or by chemical nickel-plating in
such a wave that an electrical contacting results to aluminum contacts of
the sensor wafer that are situated inside the sensor stack. Subsequently,
an intermediate space situated between the capping wafer and the sensor
wafer is filled with a polymer, an external side on the capping wafer of
the sensor stack is provided with an electrical insulating layer, and
subsequently an electrical outer contacting of the sensor stack to the
outer insulating layer is provided. For this purpose, the insulating
layer in the area of the internal electrical contacting is etched free to
the inner aluminum contacts and is filled with a metal that extends up to
the external side of the insulating layer, forming a plate. A solder bump
is placed on each such plate in order to enable the sensors to be
soldered onto a circuit board.

[0007] Here it is problematic that an electrical through-connection of the
sensor stack is not set up until after bonding of the sensor wafer to the
capping wafer. If a process of filling the vias is not successful, the
entire sensor stack, or at least a part thereof, must be scrapped.
Furthermore, in this method it is not possible to produce an individual
capping wafer having a through-connection, so that such a method also
cannot be outsourced to a supplier. In addition, it is not possible to
fill the vias using an electroplating method, so that the selection of a
material for the through-connection, and for a method of filling it, is
limited.

[0008] Therefore, an object of the present invention is to indicate an
improved method for producing a capping wafer and/or a sensor stack. In
addition, an object of the present invention is to indicate an improved
capping wafer, an improved sensor, and a configuration of a sensor
according to the present invention on a substrate. Here, according to the
present invention it is intended to the possible to provide a sensor
directly on a substrate without an additional housing, the sensor
nonetheless being resistant to environmental influences. Here, a capping
wafer of a sensor stack is intended to be capable of being produced
completely separately. In addition, an electrical through-connection of
the capping wafer is to be producible using an electroplating method. In
addition, according to the present invention the reject rate during the
production of a sensor is to be reduced.

[0009] The object of the present invention is achieved by a method for
producing a capping wafer, in particular a sealing glass capping wafer,
as recited in claim 1, and a method for producing a sensor stack, in
particular an inertial sensor stack, as recited in claim 6. In addition,
the object of the present invention is achieved by a capping wafer, in
particular a sealing glass capping wafer, as recited in claim 8, a
sensor, in particular a microelectromechanical sensor, as recited in
claim 12, and a configuration of a substrate, in particular a circuit
board, and of a sensor according to the present invention, as recited in
claim 14.

[0010] A separate capping wafer according to the present invention has
metallic through-connections from a front side to a rear side, these
electrical through-connections raising the positions of the electrical
sensor terminals to a level at which it is possible to produce a direct
solder connection to a substrate, e.g. a circuit board, through flipping
of a separated sensor. Here, an electrical contact pin of the
through-connection has a direct electrical terminal for a separate sensor
wafer. In addition, it is preferred that the electrical contact pin also
has an electrical terminal for the substrate.

[0011] In preferred specific embodiments of the present invention, the
relevant electrical terminal of the through-connection essentially
terminates flush with the front and/or rear side of the capping wafer, or
has a projection at the front and/or rear side and stands away from the
relevant side. Here, a respective electrical terminal can be a side
contact that essentially sits planar with the relevant side, a stop
contact, or a solder bump. In preferred specific embodiments of the
present invention, the electrical through-connection of the capping wafer
according to the present invention is produced by an electroplating
method. Here it is possible to also form an electrical terminal using the
electroplating method; this preferably holds for the electrical terminal
that later electrically contacts a relevant electrical terminal of the
sensor wafer. In this way, an electrical contact in the form of a press
contact or pressure contact is obtained.

[0012] A sensor stack according to the present invention is preferably
obtained in that the capping wafer according to the present invention is
mechanically solidly bonded, with a material bond, to the sensor wafer by
a sealing glass bond. Here, sealing glass is preferably applied onto the
front side of the capping wafer at the relevant regions, preferably using
a screen printing method, and the sensor stack is subsequently produced
by pressing with the sensor wafer with application of heat. Here, the
electrical through-connections of the capping wafer contact the relevant
electrical terminals of the sensor wafer, which are preferably fashioned
as bonding pads on the sensor wafer, it being preferred that the relevant
electrical contacts of the capping wafer be fashioned as stop contacts
produced using electroplating. According to the present invention, before
or after the separation of the sensor stack, the sensor stack or the
separated sensors are provided with solder bumps at the relevant
electrical contacts on the rear side of the capping wafer.

[0013] The configuration according to the present invention comprises a
substrate, in particular a circuit board, and a sensor separated from the
sensor stack, the sensor being provided directly on the circuit board,
bonded mechanically solidly thereto and contacting it electrically. Here,
an electrical contact of the sensor, in particular an electrical contact
of a cap of the sensor, electrically contacts the substrate. In this way,
it is possible to transport sensor signals from a sensor device of the
sensor through the cap of the sensor to the substrate.

[0014] The electroplating required for the present invention in preferred
specific embodiments is preferably a gold electroplating. However,
according to the present invention it is possible to use some other
electroplating or some other method. In addition, it is possible to use
any electrically conductive material for the electrical
through-connection.

[0015] The method according to the present invention for producing the
capping wafer is characterized by the steps: production of contacting
vias between the front and rear side of the capping wafer, and subsequent
filling of the vias with an electrically conductive material. Here, as
stated above, the electrically conductive material can be any
electrically conductive material. Preferably, the vias are filled with an
electroplating, preferably gold electroplating. In addition, here it is
preferred that the electroplating method last long enough that at least
on one side of the capping wafer, in the regions of the vias, the
electrically conductive material stands out, resulting in an electrical
stop contact that protrudes from the relevant side of the capping wafer.

[0016] In preferred specific embodiments of the present invention, an
electrical insulating layer, in particular silicon oxide, is applied onto
or incorporated into the rear side of a silicon wafer (capping wafer)
temporally before the production of the vias. Subsequently, copper, with
a bonding layer if warranted, is applied onto this electrical insulating
layer. Parallel to the application/incorporation at the rear side of the
electrical insulating layer, an electrical insulating layer, in
particular silicon oxide, may also be applied or incorporated at the
front side.

[0017] Subsequently, in preferred specific embodiments of the present
invention a trench mask is formed on the front side of the capping wafer
for the vias and for the caverns of the capping wafer. That is, the
trench mask has corresponding openings for the vias and for the caverns.
Subsequently, the cavern openings in the trench mask are covered by a
lacquer mask. After this, a trenching method takes place by which the
vias and the caverns inside the capping wafer are fashioned. Here, it is
preferred that the caverns not be worked as far into the capping wafer as
the vias.

[0018] In preferred specific embodiments of the present invention, first
the vias are trenched on; i.e., blind holes are made in the capping wafer
down to a certain depth. Subsequently, the lacquer mask is removed and
the trenching process is continued. In this way, there result the vias
that extend completely through the capping wafer, and the caverns also
result. Here it is preferable for the trenching method to stop after the
complete formation of the vias; i.e., the caverns are made exactly as
deep in the capping wafer as are the vias in the second step of the
trenching method. This also means that in the first step of the trenching
method this method is carried out until a remaining thickness of the
capping wafer between its electrical insulating layer and the trench
floor of the resulting vias is exactly as large as a later depth of the
caverns inside the capping wafer. However, it is also possible to etch
the caverns down to a particular depth after the complete formation of
the vias.

[0019] For later problem-free functioning of the caps, the vias of the
capping wafer are electrically insulated on their respective inner walls.
For this purpose, for example a side wall passivation can be deposited on
the inside of the vias, this passivation also being fashioned inside the
vias on the rear side of the capping wafer. That is, the passivation is
also formed on the electrical insulating layer of the capping wafer
inside the vias.

[0020] In order to enable the electroplating method to be started on the
copper layer as a plating base inside the vias, the passivation deposited
on the floor of the vias, as well as the electrical insulating layer on
the rear side of the capping wafer in the region of the vias, must be
removed. That is, the passivation, including the oxide on the rear side
on the trench floor, is preferably anisotropically opened. This results
in internally electrically insulated vias that pass completely through
the capping wafer and that are closed at the end face (rear side) only by
the rear-side copper layer of the capping wafer.

[0021] Electroplating is then used to fill the vias. The vias are filled
beginning on the inside at the copper layer on the rear side of the
capping wafer, growing in the direction of the front side. Here, the
electroplating can be continued until the deposited material protrudes
from the vias and forms there an electrical contact, preferably an
electrical stop contact. Preferably, a stop contact is formed in the
shape of a segment of a sphere, so that each electrical
through-connection has a mushroom-shaped construction, seen from the
side. For the electroplating method, all electrically conductive
materials suitable for electroplating may be used; noble and semi-noble
metals are preferred due to their very good electrical conductivity.

[0022] After the electroplating, the plating base or the copper layer is
removed, and a capping wafer results that can be combined with a sensor
wafer to form a sensor stack. For this purpose, the capping wafer and the
sensor wafer are bonded with one another in a known manner. In preferred
specific embodiments of the present invention, this takes place via a
sealing glass bond between the capping wafer and the sensor wafer. For
this purpose, the sealing glass is preferably applied onto the capping
wafer using a screen printing mask. However, it is also possible to apply
the sealing glass onto the sensor wafer. Here, sealing glass frames or
lips form on the capping wafer or on the sensor wafer.

[0023] Subsequently, the capping wafer is bonded onto the sensor wafer,
resulting in a sensor stack according to the present invention. The
sensor stack can now be separated to form sensors, and each sensor can be
provided with solder bumps on its rear side in the region of the
electrical contacts resulting from the filled vias. According to the
present invention, it is also possible before the separation of the
sensor stack to provide the solder bumps in the region of the respective
electrical rear-side contacts on the sensor stack, and only then to
subsequently separate the sensor stack into sensors.

[0024] In preferred specific embodiments of the present invention, an
electrical contacting of the sensor wafer and the capping wafer takes
place immediately through electrical stop contacts, preferably fashioned
as bonding pads, of the capping wafer, and through the electrical
terminals of the sensor wafer that stand out from the filled vias. The
electrical stop contacts sit directly on the electrical terminals of the
sensor wafer and contact them electrically.

[0025] After the provision of the solder bumps on the sensor, or the
separation of the sensor stack provided with solder bumps to form
sensors, the sensor capped according to the present invention can be
flipped on the substrate, e.g. the circuit board or a ceramic, and
soldered directly onto the substrate.

[0026] According to the present invention, it is possible to solder a
sensor, in particular a microelectromechanical sensor, onto a substrate,
in particular a circuit board, as a "naked chip" without an additional
(plastic) housing, the sensor nonetheless remaining resistant to
environmental influences. The completely separate production method made
possible by the present invention for a capping wafer according to the
present invention has advantages from the point of view of production
technique. In particular, the separate processing of the capping wafer
achieves a higher yield. If a filling process of the vias is not
successful, then according to the present invention only the cap or the
capping wafer, and not the entire sensor or an entire sensor stack, has
to be scrapped. In addition, the present invention makes it possible to
provide an electrical through-connection for the sensor or the capping
wafer using an electroplating method.

[0027] In addition, according to the present invention it is possible to
carry out the process of filling the vias, or the entire process of
producing the capping wafer, in a separate line or foundry. Many
manufacturers of microelectromechanical sensors and/or semiconductor
production plants do not have electroplating facilities, and do not wish
to acquire them due to concerns about contamination. If the
electroplating method is carried out exclusively on the capping wafer,
the production of the capping wafer can be outsourced unproblematically,
because the relevant suppliers thereby gain information only about the
capping wafer, and do not learn any further company secrets regarding the
sensor and its manufacture.

[0028] Further specific embodiments of the present invention result from
the remaining dependent claims.

[0029] In the following, the present invention is explained in more detail
on the basis of exemplary embodiments with reference to the accompanying
drawing. In the drawing,

[0030] FIGS. 1 through 8 show a production method according to the present
invention for a capping wafer according to the present invention,

[0031] FIGS. 8 through 10 show a production method according to the
present invention for a sensor stack according to the present invention,
or a sensor according to the present invention, and

[0032]FIG. 11 shows the sensor according to the present invention
provided on a substrate.

[0033] When in the following reference is made to a capping wafer or to a
sensor wafer, in each case the individual part, i.e. a capping wafer or a
sensor wafer in itself, is meant. If the two wafers are affixed to one
another, this is no longer referred to as a wafer, but as a sensor stack.
In addition, in a capping wafer at least one individual cap is fashioned,
or the capping wafer has at least one individual cap. Analogously, in the
sensor wafer there is fashioned at least one sensor element, or the
sensor wafer has at least one individual sensor element.

[0034] According to the present invention, a sensor can be soldered onto a
substrate as a "naked chip" that does not have an additional housing but
is nonetheless insensitive to external influences. Preferred for this
purpose is a monolithically integrated sensor having low-ohmic electrical
connections to the exterior. In such a case, the high-ohmic electrical
connections are already in themselves shielded by an integration of the
sensor to such an extent that attention to this point is no longer
required during the construction and connection design. The electrical
inputs and outputs of a monolithically integrated sensor are electrically
stable, are not susceptible to disturbance, and are loadable. A direct
connection via the substrate to other electrical or electronic components
is therefore easily possible. The omission of the housing according to
the present invention not only enables cost savings, but also reduces the
dimensions of the sensor and a thickness of the sensor chip, or a
thickness of the substrate, by a significant amount. In addition to an
encapsulation, such a monolithically integrated sensor includes a sensor
device and an integrated circuit, fashioned for example as an
application-specific integrated circuit (ASIC).

[0035] Such a procedure is possible through the use of a cap construction
according to the present invention, preferably likewise bonded onto a
sensor element using a sealing glass as in the prior art. With the use of
such a currently existing capping design, flipping is not possible,
because bonding pads are situated in recessed fashion and can be accessed
only through openings, and/or no separate filling of vias is possible.

[0036] On the basis of FIGS. 1 through 7, or FIGS. 1 through 8, in the
following a manufacturing method according to the present invention of a
capping wafer 100 according to the present invention is explained in more
detail, in which at least one cap 10 is formed, shown in a
cross-sectional view in the respective Figure. Preferably, however, such
a capping wafer 100 has a large number of caps 10.

[0037] Capping wafer 100 is provided, before a bonding process, with
electrical through-connections from a front side 101 to a rear side 102,
a complete processing, i.e. production of capping wafer 100, being
completely concluded before the actual bonding process. For problem-free
functioning of cap 10 resulting from capping wafer 100, it is necessary
for the through-connections to be electrically insulated from one another
and from a substrate of capping wafer 100. In the method according to the
present invention, this is ensured by the fact that inner walls 112 of
contacting vias 110, as well as front side 101 and rear side 102 of
capping wafer 100, are coated with an electrical insulating layer 103,
106, 111, preferably made of silicon dioxide 52. An actual electrical
through-connection of capping wafer 100 is then an electrically
conductive material 51, e.g. deposited in an electroplating process, such
as a metal 51, preferably gold 51.

[0038] A production method for capping wafer 100 may proceed as follows.
Referring to FIG. 1, the method begins on rear side 102 of capping wafer
100, with a deposition or thermal growth of electrical insulating layer
106, which is preferably made of silicon oxide 52. In the case of a
thermal growth of oxide, capping wafer 100 can advantageously even be
provided with oxide layer 52 on front side 101 and on rear side 102
simultaneously; see also FIG. 2. In this way, the later deposition of
electrical insulating layer 103 on front side 101 of capping wafer 100
can be omitted (see below). Like the following electrical insulating
layer 111 (see FIG. 5), preferably also made of silicon oxide 52, these
insulating layers 103, 106 are intended to ensure the electrical
insulation of electrical contacts 131, 132, 133, or of electrical contact
pins 130, of capping wafer 100 (see also FIGS. 9 and 10).

[0039] Subsequently, on rear side 102 a metal 53, in particular copper 53,
is sputtered on or is applied in some other way over a large surface as a
starting layer for an electroplating method; this is also shown in FIG.
1. For the case of sputtering on of copper 53, it is preferable that the
copper adhere to the silicon oxide 52 with an bonding layer, e.g. made of
Cr, WTi, etc.

[0040] On front side 101 of capping wafer 100, a mask is required for a
deep trench process (see FIG. 2), which makes it possible, preferably
without a new lithography, to carry out two trench processes having
different depths, as is shown in FIGS. 3 and 4. Such a trench process is
for example a deep-structuring method such as a trench etching. The first
trench process is required in order to produce caverns 120; the second is
required in order to etch contacting vias 110 through capping wafer 100.
In the following, contacting vias 110 are referred to only as vias 110.
Caverns 120 are required for sensor devices 220 (see FIGS. 9 through 11)
of sensors 2 that later result, in order to better enable the setting of
an internal pressure inside the relevant sensor 2. Caverns 120 may be
omitted in certain sensors 2.

[0041] For sensors 2 having caverns 120, a hard mask 103 or trench mask
103 is produced, as is shown in FIG. 2. This takes place for example by
deposition of an oxide 52 on front side 101 of capping wafer 100, or by
using a thermal oxidation of rear side 102 of capping wafer 100, this
thermal oxidation simultaneously producing oxide 52 required for mask 103
on front side 101 as well (see above). There subsequently takes place a
lithographic and etching-based structuring of mask 103, whereby mask 103
receives both sorts of etching openings 104, 109. Here, etching openings
104 provided as through-openings 104 in mask 103 are provided for the
trenching of vias 110, and through-openings 109 are provided as etching
openings 109 for the trenching of caverns 120.

[0042] Through-openings 109 of a sort provided for the trenching of
caverns 120 are covered with a lacquer mask 105, a so-called photoresist
105; this can be accomplished by applying a photoresist 55 and subsequent
(photo-)lithography. After the mentioned lithography step,
through-openings 104 of the other sort, provided for the trenching of
vias 110, are again free of photoresist 55 and oxide 52.

[0043] After the first trench process, only vias 110 are etched, because
through-openings 104 are covered neither with photoresist 55 nor with
oxide 52. Subsequently, photoresist 55 situated on through-openings 109
is removed (see FIG. 3), and the trench process is subsequently
continued, which is shown in FIG. 4. Here, caverns 120 and a continuation
of vias 110 are etched. A difference in the etching depths of caverns 120
and vias 110 can be controlled via the time of removal of photoresist 55.
The trench for the electrical through-connection of capping wafer 100 is
stopped on the rear-side oxide. Here, overetching without damage is
possible, and caverns 120 can then be etched further down to a desired
target depth. To this extent, it is also possible first to completely
finish producing vias 110, then subsequently to remove photoresist 55,
and only then to produce caverns 120 in capping wafer 100.

[0044] If vias 110 and caverns 120 are provided in capping wafer 100, a
conformal oxide deposition process is preferably required, which coats
inner walls 112 of vias 110. TEOS ozone deposition is well-suited for
this purpose. Here, electrical insulating layer 111 shown in FIG. 5
results, which is also referred to as a side wall passivation of vias
110. Before the subsequent electroplating shown in FIG. 7, oxide 52,
preferably silicon dioxide 52, on trench floors 113 of vias 110 still has
to be removed down to metal 53, or down to the bonding layer (CR, WTi,
etc.) of copper 53, preferably using an anisotropic etching process,
which layer can then likewise be selectively worn away down to metal 53.
The latter can be accomplished using wet etching solutions that
selectively etch Cr or WTi relative to copper 53.

[0045] The electroplating process is now started in vias 110 using copper
53 as the plating base; through this process, vias 110 are filled with an
electrically conductive material 51, preferably gold 51, beginning on
copper layer 107 up to front side 101 of capping wafer 100. If the
electroplating process is controlled in such a way that after the filling
of vias 110 there is an overgrowth of gold 51 on front side 101 and at
the sides, stops 131, preferably hemispherical in shape, are obtained,
which, in a subsequent bonding process, are placed on bonding pads 231 of
a sensor wafer 200 (see FIGS. 9 through 11) in order in this way to
produce electrical contacts that are fashioned for example in the form of
electrical press contacts or pressure contacts. For good electrical
contacts, and in order to prevent later corrosion, bonding beds 231 of
sensor 2 or of the ASIC are likewise coated with gold 51 or are made of
gold 51, so that gold 51 on gold 51 contacts result. The end of the
electroplating process is shown in FIG. 7.

[0046] Before an application of sealing glass 54 onto capping wafer 100
prepared in this way, and the execution of a bonding process between
capping wafer 100 and sensor wafer 200, on the rear side of capping wafer
100 electrically conductive layer 107 still has to be removed selectively
to gold 51 and electrical insulating layer 106 (see FIG. 8). Sealing
glass' 54 is applied onto capping wafer 100 in the form of sealing glass
lips 140 or sealing glass frame 140. According to the present invention,
it is also possible to apply sealing glass lips 140 or sealing glass
frame 140 onto sensor wafer 200. In the depicted exemplary embodiment of
the present invention, the respective caverns 120 with the relevant
sensor devices 220 are separately encapsulated relative to the relevant
electrical vias as well, two circumferential sealing glass frames 140
being provided for each resulting sensor 2. However, it is also possible
according to the present invention to provide only one of these two
sealing glass frames 140. It is not necessary according to the present
invention to seal sensor device 220 in fluid-tight fashion. Thus, it is
possible to configure sealing glass frame 140 to be open, and to provide
corresponding sealing glass lips 140 only at suitable locations.

[0047] Sealing glass 54 is preferably applied onto capping wafer 100 using
a screen printing method. Alternatively to a screen printing method,
sealing glass strips 140 can also be provided using stencil printing or
some other coating process. In addition, instead of sealing glass 52
other materials may also be used that are capable of bonding two wafers
100, 200 to one another, preferably in gas-tight fashion; for example,
this can take place using adhesives, thermoplasts, or other plastics or
solders.

[0049] After the bonding process, sensor stack 1 is ready for separation;
in order to be capable of being soldered, individual sensors 2 must be
provided with solder bumps 133. This can also take place before the
separation of sensor stack 1. Solder bumps 133 are placed directly onto
electrical contacts 132, situated on rear side 102 of sensor 2 or of
sensor stack 2, of contact pins 130. That is, in preferred specific
embodiments of the present invention solder bumps 133 are placed directly
onto gold 51 of electrical contact pins 130, and can be made-for example
of a lead-free AgSnCu solder 56. Of course, other solder materials 56 may
also be used. Alternatively, solder bumps 133 may also be placed onto a
substrate 3, in particular a circuit board 3 or a ceramic 3 onto which an
individual sensor 2 is to be soldered. Sensor 2 soldered to substrate 3
is shown in FIG. 11.

[0050] If the electrical through-connection is too small in a diameter,
and/or a wetting with solder bumps 133 is difficult, then a structured,
electrically conductive layer, in particular a gold layer 51, which
determines wetting surfaces for solder 56, can be applied, directly at
the beginning of the method according to the present invention, on rear
side 102 of capping wafer 100 under copper 52, i.e. between electrically
conductive layer 107 and electrically insulating layer 106. This results
in an enlarged contact surface of solder bumps 133 with electrical
contact pins 130. In addition, this step can be used to move the position
of electrical contacts 133, i.e. of solder bumps 133, as it were to
"rewire" them, or to define sawing lines and/or alignment marks for
sensor stack 1.

[0051] The present invention is applicable to all sensors 2, in particular
motor vehicle sensors 2. In particular, the present invention is
applicable to all monolithically integrated sensors 2 that in addition
also require a preferably hermetically sealed cap 10. These are in
particular (surface) microelectromechanical sensors 2, inertial sensors 2
for measuring accelerations and/or rates of rotation, such as rotational
rate sensors 2 or acceleration sensors 2, and also resonators 2.